Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor 13 compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts.
Particular problems occur with regard to the propulsive fan of the gas turbine engine. It will be appreciated that the propulsive fan draws air into the engine under low pressure and this air is propelled typically at an angle towards stator vanes within the engine in order to straighten the airflow. Unfortunately, as a result of several factors, there are problems associated with this basic relationship. Firstly, due to the nature of attaching a gas turbine engine to an aircraft wing, there is a so-called droop phenomena whereby the airflow is not presented parallel to the engine axis. This input effect upon presentation of the air flow to the air intake may be further compromised by side winds etc. Further problems relate to aerodynamic losses as a result of fan blade tip vortex phenomena and other factors. It will also be understood that there is a significant problem with respect to so-called fan forcing which is as a result of circumferential pressure variations around the air intake of the gas turbine engine such that each fan blade must force its way between these variations and this in turn causes stress to the fan blades. A further problem relates to the necessity to mount the gas turbine engine with a substantial mounting pylon just to the rear of the stator vanes and also provision of radial drive struts at similar positions. The pylon and radial drive struts further distort the laminar flow through the stator vanes. Typically, a pylon and/or radial drive struts will create a back pressure which acts prior to the stator vanes in order to further distort uniform laminar flow across the air intake for a gas turbine engine. It will be appreciated that there may be other forms of obstruction behind the vanes which create a back pressure.
Previously, attempts have been made to ameliorate the effects of back pressure due to pylons and radial drive struts to provide an equalisation of laminar flow presented through those stator vanes for the fan blades. In short, by either cyclic angle stagger, that is to say presenting the same type of stator vane but staggered at different angles relative to the flow, or by cyclic camber, that is to say different shaped vanes at different positions, laminar flow has been adjusted to achieve approximate uniformity across the region immediately upstream of the stator vanes. It will be appreciated by varying the vanes either by cyclic stagger or cyclic camber techniques, it is possible to adjust for the back pressure effects of pylons and/or radial drive struts. Nevertheless, specification of the appropriate cyclic stagger or cyclic camber variations in the stator vanes requires a detailed analytical model of airflow through the gas turbine engine fan inlet. This detailed analysis is complicated due to the high number of possible variables such that previously, modelling has been limited to the angular laminar flow presented by rotation of the airflow aerofoil blades of the fan to the stator vanes given particular pylon and/or radial drive strut configurations. In such circumstances, the most significant problem is that relating to fan forcing, the stator vanes positions have been optimised in order to minimise that problem such that droop/crosswinds have been neglected and such as other efficiency reducing problems aerodynamic losses and buffet flutter have been accepted as inherent operational consequences. Nevertheless, all these effects are modified by the presence of droop and/or crosswinds.